The invention pertains to photoconductive semiconductor devices, more particularly to those that are compatible with monolithic semiconductor fabrication technology, and most particularly to those compatible with gallium arsenide technology.
Photoconductive semiconductor devices have widespread application in electronics. With the growing importance of gallium arsenide technology, most especially for high frequency applications, there is an ever growing need for effective photoconductive devices that are compatible with techniques for fabricating monolithic semiconductor devices in general, and gallium arsenide monolithic devices in particular. A general semiconductor photoconductor can be in a form analogous to a field effect transistor, i.e. having a source and a drain d.c. biased with respect to one another, and separated by a current carrying channel. Light incident on the channel creates electron-hole pairs which alter the channel's resistance, and hence the current flowing in the channel under the d.c. bias. This channel current can be thought of as having two components: signal current (I.sub.s) resulting from the interaction of photons with the semiconductor channel as discussed above, and dark current (I.sub.d) which exists independent of photo illumination, and is a quiescent current. The respective magnitudes of dark and signal current are given by the following relations: EQU I.sub.d =nv.sub.sat L(t-W.sub.D) EQU I.sub.s =(qP.lambda.G.eta.)/hc
where n is the doped carrier concentration in the channel, v.sub.sat is the saturation velocity of carriers in the channel, L is the channel width, t is the channel thickness, W.sub.D is the surface carrier depletion in the channel, q is the magnitude of carrier charge, P is optical power incident on the channel, .lambda. is the wavelength of light incident on the channel, G is the photoconductor's gain, h is Planck's constant, c is the speed of light, and .eta. is the quantum efficiency of the semiconductor channel, a measure of the number of free carriers contributing to signal current I.sub.d generated per incident photon.
As can be seen from the first equation, I.sub.d is proportional to doping concentration n. Since the quantum efficiency .eta. is dependent upon the thickness of the sensing layer (t - W.sub.D), Is is proportional to the thickness of the layer. However, in gallium arsenide the depth of depletion W.sub.D and doping level are not independent because the surface of gallium arsenide has an inherent potential (about 0.7 volts) which creates the depletion layer W.sub.D by driving carriers away from the surface into the bulk crystal. For many doping levels of interest, the depth W.sub.D of the depletion layer is of the same order as the depth of photon penetration, thus rendering such devices virtually useless, or, at best, highly insensitive (low signal current I.sub.s per incident photon). Increasing doping n decreases W.sub.D (hence increases signal current I.sub.s) by providing additional charge to counterbalance the surface potential. Unfortunately, this also drastically increases dark current I.sub.d. Because dark current I.sub.d depends strongly on doping in the bulk channel volume, increasing signal current I.sub.s in this manner disproportionately increases dark current I.sub.d. Thus, the apparent inability to increase signal current I.sub.s independent of dark current I.sub.d makes photoconductive devices of this kind apparently impossible to optimize.